Many kinds of systems have been disclosed which monitor and/or control traffic. Some simply operate regular traffic signals or warning signs U.S. Pat. No. 4,908,616 discloses a simple system deployed at a traffic signal controlled intersection. A warning device positioned in the approach to the intersection at a "reaction point" gives an indication to a driver as to whether or not his/her vehicle is too close to the intersection to stop safely if the traffic signal has just changed. The system does not measure vehicle speed and cannot account for differing stopping distances for different classes of vehicle.
Systems which measure the speed of the vehicle include that disclosed in U.S. Pat. No. 3,983,531 which measures the time taken for a vehicle to pass between two loop detectors and operates a visual or audible signal if the vehicle is exceeding a set speed limit. U.S. Pat. No. 3,544,958 discloses a system which measures the time taken for the vehicle to traverse the distance between two light beams and displays the measured vehicle speed on a warning sign ahead of the vehicle. Conversely, U.S. Pat. No. 3,275,984 discloses a system which detects when traffic is moving slowly, thereby indicating that a highway is becoming congested, and activates a sign near a highway exit to divert traffic via the exit. More recently, U.S. Pat. No. 4,591,823 disclosed a more complicated system using radio transceivers located along the roadway which broadcast speed limit signals by transceivers carried by passing vehicles. Signals returned by the vehicle mounted transceivers enable the roadside transceivers to detect speed violations and report them to a central processor via modem or radio.
Traffic monitoring systems have also been disclosed which monitor various parameters of the vehicle itself to enable the class of vehicle to be determined. Thus, U.S. Pat. No. 5,173,692 discloses a system for controlling access through a gate or entrance according to class of vehicle and which uses ultrasonic detectors to detect vehicle profiles and compare them with established profiles to determine the class of vehicle. U.S. Pat. No. 3,927,389 discloses a system which counts the number of axles on a vehicle to enable classification of the vehicle and calculation of an appropriate tariff for use of a toll road.
Systems are also known which use sensors to weigh vehicles while they are in motion so as to detect, for example, overweight commercial vehicles. Examples of such systems are disclosed in U.S. Pat. Nos. 3,835,945; 4,049,069; 4,560,016 and 4,793,429. U.S. Pat. No. 5,008,666 discloses traffic measurement equipment employing a pair of coaxial cables and a presence detector for providing measurements including vehicle count, vehicle length, vehicle time of arrival, vehicle speed, number of axles per vehicle, axle distance per vehicle, vehicle gap, headway and axle, weights.
The known systems do not, however, deal with the fact that a particular site will not be a hazard for one type of vehicle, for example a car, but will be a hazard for, say, a heavily-loaded truck. When commercial vehicles, especially large trucks, are involved in accidents, the results are often tragic. Statistics show that, although commercial vehicles are involved in a relatively small percentage of all motor vehicle accidents, they are involved in a higher percentage of fatal accidents than other vehicles. Consequently, they warrant special monitoring.